Dyspnea is the chief patient complaint in a variety of diseases of the pulmonary system. These diseases include chronic bronchitis (12.5 million US patients), emphysema (1.7 million US patients), and asthma (18 million US patients), collectively referred to as Chronic Obstructive Pulmonary Diseases, or COPD. Dyspnea is also reported by patients suffering from combinations of the foregoing diseases, and/or other pulmonary diseases, and non-pulmonary diseases (notably in heart failure). Dyspnea, while a common medical term, is actually poorly defined and ultimately subjective since it is generally the perception of difficulty breathing or difficulty catching one's breath, and more generally, an uncomfortable sensation of breathing.
The severity of pulmonary diseases can typically be measured using objective techniques, such as FEV1 (the patient's forced expiratory volume in the first second of exhalation), minute ventilation (the volume inhaled or exhaled by the patient in one minute), arterial blood gas levels (e.g., of oxygen or carbon dioxide), among others. By contrast, the patient's dyspnea experience can be simply one of difficulty breathing, ultimately leading to a reduction or elimination of physical activity due to this discomfort. That is, the patient complaint is of dyspnea and a loss of mobility or physical function, not of a decreased FEV1.
In many ways dyspnea can be analogous to the perception of pain. While an organic source of the pain may be present (a broken bone, for example), the pain itself can be a problem and may require palliative treatment. Furthermore, in the same way that an individual can suffer from chronic pain for which an organic cause is either absent or inadequate to cause the pain, some patients can suffer from severe dyspnea despite relatively normal objective measures of pulmonary performance.
The origins of dyspnea remain unclear. Studies and experience have yielded confusing and often seemingly contradictory results. Treatments for dyspnea range from supplemental oxygen therapy to sitting in front of a fan to systemic opiates. Furthermore, dyspnea can be experimentally induced by vigorous exercise, breath-holding, breathing through a restrictive mouthpiece, or breathing carbon dioxide in symptomatic pulmonary disease patients. A common, though unproven theory, is that dyspnea derives from a mismatch between outgoing motor signals to the respiratory muscles and incoming afferent information. In one example, under a give set of conditions, the brain can expect a certain pattern of ventilation and associated afferent feedback. Deviations from this pattern can cause or intensify the sensation of dyspnea.
While dyspnea is often the chief complaint of a patient, there is currently no pharmacologic agent that primarily treats dyspnea. That is, a variety of bronchodilators are used to treat asthma and other COPD, and while they demonstrably increase FEV1, their effects on dyspnea can be modest and can fall below that of clinical significance. Accordingly, there remains a need for methods and devices that effectively treat dyspnea.